


Overflow

by MeGaLoTrash



Category: Undertale (Video Game)
Genre: Dirty Talk, Dry Humping, Ecto-Penis (Undertale), Ecto-Tongue (Undertale), Ecto-Vagina (Undertale), Explicit Sexual Content, Fluffy Smut, Fontcest, Hand Jobs, Heavy Petting, Hidden Realtionship, Loud Sex, Love Confessions, M/M, Magic used during sex, Oral Sex, Overstimulation, Semi-Public Sex, Sensitive bones, Soul Sex, Wall Sex, Waterfall (Undertale)
Language: English
Status: Completed
Published: 2018-07-13
Updated: 2018-07-13
Packaged: 2019-06-09 23:06:46
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 5,207
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/15278193
Author URL: https://archiveofourown.org/users/MeGaLoTrash/pseuds/MeGaLoTrash
Summary: Sans would do anything for his brother, even more so now that they're lovers, but he's struggling with the restrictions that come with keeping things secret. Could Papyrus's proposal for a 'not-date' in public be what they both need right now?Shameless super fluffy smut based on a conversation with @redtomatofan about sex in a waterfall... ;)





	Overflow

Sans waved sheepishly at the two young monsters as they walked past him. He recognized them from his hot dog stand in Hotland and even though he really didn’t want to see anyone tonight it would be more suspicious if he listened to his instincts to run and hide. However, he couldn’t resist the urge to check behind him to make sure that they weren’t watching him when they passed by. He knew he was being paranoid when all he saw was the lion monster casually lacing their fingers with the blushing bunny. Sans was happy for them, he knew that the lion had been pining over his date mate for months, but he couldn’t ignore the way it made his soul ache. Not for himself though, but for the momentary twitch in his brother’s smile when he looked back to see what had Sans’s attention.

It wasn’t like he could take his brother’s hand or pull him down for a kiss even if it was what they both really wanted. 

Which was why Sans had initially been reluctant to go to Waterfall with his brother tonight. Papyrus had never actually said the word date but it was obvious from everything that was currently building up between them. They had been dealing with their not-so-brotherly feelings separately for years, known about how the other felt for months, but had only started acting upon them a few weeks ago. The problem wasn’t how they felt though. It was obvious they were both just as into each other from the feverish kisses would turn into desperate grinding and eventually fucking with hands clamped over their mouths. That was the real issue; they were both so terrified of being caught together that it put restraints and barriers on their time together.

Sans should have known that they couldn’t keep going the way they were. Since he didn’t have a solution to how they could be together without restrictions he preferred to bury his head in the sand and pretend things were fine. It was bad, but it was better than the alternative of them ending their romantic relationship. Being around Papyrus was hard enough when he had been trying to keep his feelings to himself. But now that he knew Papyrus felt the same as he did, now he’d been able to kiss away his tears and know the feeling of being buried inside him, he knew they couldn’t go back to being just brothers. He was pretty sure Papyrus felt exactly the same. He had been the one to break under the pressure first and set the events of tonight in motion after all. 

Papyrus insisted that they were going out when Sans had returned from his sentry post earlier that evening. With that determined look that Sans knew was useless to argue against. How could he say no when he spotted the perfectly packed picnic basket on the table behind him? Papyrus wasn’t an idiot, he knew that they couldn’t do anything that couples did in public just as much as Sans did. A picnic wasn’t something inherently exclusive to couples though. Even if it was at dusk, in the underground’s most romantic setting, with enough dimly lit areas to steal a quick kiss or something more…

Sans knew as soon as he agreed that he was fucked. He tried to reassure himself that no one would have any inclination to know what was really going on between them unless they fucked up massively.it didn’t help his desire to do more with Papyrus, but it did reassure him that monsters wouldn’t know just by looking at them. To everyone in the underground they were just the quirky skeleton brothers who didn’t always follow the rules of normal social conventions. An intimate picnic for two would only be seen as inappropriate if they let it appear that way. It took the walk to Waterfall for Sans to convince himself that it wouldn’t be that bad, in fact he had even let himself get excited. Being out on a date was something so simple, something that most couples took for granted, but it meant the world that it was something Papyrus wanted to do with _him_.

To his credit Papyrus had picked a secluded spot. He laid their picnic blanket down next to part of the river; just far enough away from the path for them to be able to see anyone coming, but not so far in the darkness that monsters wouldn’t be able to make out what they were doing. Waterfall had been pretty lively when they had first got there with monsters coming home from work or enjoying an evening stroll in the marsh, but their encounters became few and far between as time went on. Sans had been overly tense at first, the back of his skull producing more sweat than was normal even with it being a warm evening. Seeing that no one noticed or cared helped him relax though. Papyrus chatted away like nothing was wrong with anyone who passed and Sans could feel himself start to relax into it. He even shared a few of his worst jokes with anyone who would listen, much to Papyrus’s dismay. 

It was nice just to be together in public again and not spend the entire time worried about getting caught. 

The rest picnic had gone by without an incident; they each ate a passable plate of Papyrus’s spaghetti, split a cherry flavoured bisicle, and packed up their box with only a few lingering looks while they folded the blanket. Sans’s soul was still fluttering happily as they walked home. Papyrus was leading a step or two ahead and taking them on a longer route than usual, chatting away about new puzzle techniques he’d learnt and a new pasta dish he wanted to try. Sans didn’t mind. He wanted to get home so he could show Papyrus just how much he appreciated tonight, but he didn’t want their date to end anytime soon either. Walking a little out of their way to get home was a compromise he was happy to make. 

Since their romantic relationship was so new they had been spending almost all of their time together doing all the things they had been resisting for so long. To Sans every moment with Papyrus was incredible. It as just as nice to spend time enjoying each other’s company as it was slowly rocking into him when they were in bed. Sans realised just how much he’d been missing Papyrus’s tall tales or making struggle not to laugh at his terrible jokes. Even when they had been quietly enjoying the scenery together he had taken the time to study Papyrus’s face and remember why it was worth doing all this sneaking around. 

_and remember why he’d fallen in love with his brother in the first place._

Sans had to stop himself mid chuckle so he didn’t choke on the unexpected thought. He knew Papyrus loved him, he knew that there was so much more than lust between them, but he hadn’t admitted that he was _in love_ with him yet. Not even to himself. It seemed stupid to deny when every fibre in his bones was screaming at him to get closer to his brother, fuck all the consequences, but he was afraid. Afraid of coming on too strong and afraid of what it meant if Papyrus was in love with him too. Love was meant to be something amazing; it was something that could strengthen a soul, heal wounds, or even create life. They could never have that though, not when everything was this fucking complicated. 

It took Sans a few moments to realise they had stopped, being far too caught up in his thoughts to have even heard if Papyrus had said something as to why. It didn’t look that way though. Papyrus was just as lost in his thoughts but he seemed puzzled, tapping his mandible as he looked around. Sans took proper notice of where they were for the first time then and realised that they were nowhere near the route back to their house. Papyrus hadn’t been wandering the long way home, he was definitely taking them somewhere specific. Before Sans had the chance to ask Papyrus was struck with a thought and reached back to grab Sans by the arm. 

Sans went along with it, only to have Papyrus drag him into part of the river. 

“hey um- _water_ we doing paps?” Sans tried to keep it light-hearted even if his mind was trying to tell him to run. 

Not that it would have done any good. This part of Waterfall had a strong current due to the, heh, actual waterfalls. If Sans let go of Papyrus he was sure he’d float down to the garbage dump like the trash he really was. 

“I’m Looking For Something Important Brother,” Papyrus said while tightening his grip on Sans’s arm “Do Try And Keep Close, I Need To Concentrate.”

Sans did as he was told and kept quiet for a moment. He thought about checking around for anyone in the area, but he knew standing in the middle of the river was a lot easier to explain than their not-date. So he didn’t bother, choosing to focus on Papyrus instead. His brother still looked puzzled but there was something that Sans had somehow missed before. His cheekbones were coloured with a strong orange blush, far too reminiscent of the look Papyrus had when they were intimate. In the twilight world of Waterfall it was far too easy for Sans to slip into memories of Papyrus under him and begging for more with his eyes. It felt like all the magic in Sans’s body was headed straight to his pelvis, swirling around his bones fast enough to make him feel lightheaded. 

_what the hell was his brother planning?_.

“i don’t mean to _rain_ on your parade bro but- um-” Sans knew he was wavering, he could feel his soul pounding just from being close to his brother. “don’t-don’t you think we should head home before we become all _washed up?_ ”

“I Swear One Day I’m Just Going To Disown You Sa-” Papyrus stopped mid-sentence as his gaze flicked to a point behind Sans “There!” 

Papyrus had apparently found what he was looking for and was intent on dragging Sans with him. By the looks of things they were headed straight for one of the waterfalls which could only mean that Papyrus had actually lost his mind and wanted to bring his brother down with him. _fucking perfect_. Sans screwed his sockets shut and braced himself for impact but it never came. He heard the sounds of the waterfall raise to a crescendo as they got closer, felt the rush of water hammering down on his body, but now there was nothing. Wherever they were felt dry, it felt warm. Sans hesitantly opened an eye socket to find a giddy looking Papyrus smiling at him in what appeared to be a hidden cave. It was dimly lit from the solitary mushroom light that made them cast shadows on the sparkling cave walls. 

When Sans looked back to Papyrus he felt like all the air was being sucked out of his body. Papyrus looked more beautiful than he had ever seen him before, like the faux stars on the walls were sparkling just for him. 

“Isn’t It Amazing!” Papyrus beamed as he gestured around the cavern, the smile on his face growing at the same pace as his blush. “I Found It When I Was Waiting For Undyne One Day And I- Mffhfmmm”

The end of Papyrus’s sentence was cut off when he turned towards the waterfall, but Sans didn’t miss the way the magic in his face deepened even further. That coupled with the soft glow coming through his brother’s wet clothing made Sans’s mind race. Papyrus had brought him here for a reason. _a ‘naughty’ reason._ Even though Sans had an idea what it might be he needed to be completely sure before he threw himself at his brother. 

“sorry paps, i couldn’t hear over the-”

“That’s The Point Sans, It’s Perfect!” Papyrus exclaimed as he wavered between being proud of himself and his bashful nature, making Sans’s soul feel like it was gonna hammer itself clean through his spine. “I- Mean- I Um, Brought Us Somewhere Noisey, So We Can Be… As Loud As We Like While We Are Together- _Intimately_ \- If You Want To That Is!”

It felt like the warm air in the cavern skyrocketed suddenly while the atmosphere around them shifted. Since they weren’t really able to talk frankly about their sexual relationship for fear of slipping up around others, this was the most direct Papyrus ever been since they had initially confessed their attraction. It was more of a turn on that Sans could have ever imagined. His cock had formed, he couldn’t have stopped it if he tried, and it was throbbing heavily against the seam of his shorts. Papyrus had not only come up with an ingenious solution to their problems, but the semi-public setting was kinky as fuck. Sans went through all the emotions of falling in love with his brother for the second time that evening in a single moment. 

“oh pap…oh fuck-” Sans tried to hold back a groan as he approached his brother but he could already feel his carefully constructed restraint crumbling around him. “this is- it’s fucking brilliant.”

Papyrus scoffed and turned his head even while he was shivering in response to Sans’s hands travelling up his arms. “Being Loud Doesn’t Mean We Have To Be Crude Sans!”

“heh, we’ll see about that,” Sans winked and pulled Papyrus’s scarf hard enough to clash their teeth together. 

Sans was in heaven as soon as he felt Papyrus’s tongue press against his. Going from having to be careful about even kissing for too long in case they got carried away to finally being able to do everything that he wanted was a little overwhelming. It felt incredible to not worry about pushing too far, to be able to kiss Papyrus while projecting all of the intense feelings he’d been holding back. Sans kept one hand firmly in his brother’s scarf while the other cupped his skull and deepened the kiss with a moan. When he heard Papyrus’s answering whine he knew that his last shred of anxiety was going to have to take a backseat. 

Tonight was going to be just about them. Nothing in the whole underground was going to stop Sans from showing Papyrus just how much he meant to him. 

Their kissing quickly evolved into heavy petting as both of their souls cried out for more. Papyrus’s breathing was coming in short gasps when Sans backed him into one of the cave walls. Papyrus’s long limbs opened naturally when Sans moved one of his hands down his chest, allowing them to slot together like puzzle pieces. He could feel the way that Papyrus’s wet T-shirt clung to his ribs and how the heat from his shorts had almost dried the material completely. Sans loved this outfit on Papyrus, the bright summer colours would have been too much on anyone else but his brother. But the shorts had to go. It didn’t matter that they were still technically in public and that keeping as much clothing on as possible was going to be the safest option, Sans wasn’t going to cut any corners in his goal to make Papyrus scream.

In an ideal world Sans would have torn the flimsy material clean off. Walking behind Papyrus in the tiny shorts had been its own kind of torture and he would have been pleased to rip them to shreds. Papyrus walking home without bottoms would certainly turn some heads though, so Sans knew that it wasn’t an option. He gently rocked his hips into Papyrus a few times when breaking the kiss, shivering at the whimpers his brother made when their concealed magic touched. He sank to his knees and started to pull at Papyrus’s shorts, cursing under his breath when he realised they were still clinging to his bones from going through the water. It was like they were the tight wrapping around the giftmas present Sans had been waiting all year for. He kept repeating the bullshit mantra of ‘good things come to those who wait’ to keep himself from losing his cool as he slowly wiggled the shorts down his brother’s legs, almost cheering when he finally got them low enough for Papyrus to step out of. 

It wasn’t every day when that he got treated to a face full of his brother’s dripping pussy. 

Papyrus was covering his mouth out of habit before Sans motioned him to move his hand away, not wanting to miss a single sound. He moved his skull closer to his brother’s magic but paused inches away so he could really appreciate just how amazing Papyrus looked. He was flushed and panting, using the wall of the cave to support his shaking frame while he watched Sans through widened sockets. Sans slowly moved his gaze downwards for effect, hissing through his teeth when he refocused on Papyrus’s pussy. His brother always formed the most incredible magic; from the way it felt inside to the tantalising way his outer lips would open when he really aroused. It was just as perfect as him, and Sans was dying for a taste. 

Oral was one of Sans’s favorite sex acts to perform, but they had only been able to try it once before realising that it made Papyrus far too loud. Sans planned to make up for lost time. He started with directing his hot and heavy breaths right onto the swollen magic, having to use one of his hands to steady Papyrus’s hips when he jerked towards him. It felt so good to be the one making his brother react this way, without restrictions, and they were barely getting started. When he’d done enough teasing he moved even closer, making sure Papyrus was looking right into his eyes when he licked that first stripe across his pussy. Papyrus’s magic was so sweet on his tongue made Sans immediately need another taste, but the hint of musky aroma unique to his brother was what he could really get drunk on. 

Sans continued to watch as his brother fell apart, gradually increasing the pace and pressure of his tongue until he felt Papyrus’s juices dripping down his chin. Papyrus was unable to keep still, his spine arching as his hips bucked against Sans’s willing tongue. But the best thing was the noises. It was so easy to map out the points that were doing it for Papyrus since he would moan and gasp at every little jolt of pleasure. Sans got lost in his task, burying his face in Papyrus’s core as he fucked his entrance with his tongue. He used the hand that wasn’t pinning Papyrus to the wall to assist him by slipping between them and drawing soft circles on his engorged clit. The reaction from Papyrus was instant as his whimpers and moans turned into actual words.

“Sans- Sans- Sans!” Papyrus’s eyes were screwed shut as he chanted his brother’s name and spiraled closer to the edge. “It’s So Good, We Need To Stop Before I-I’m- So-Hnnn-”

“no pap, i’m not stopping. in fact-” Sans shifted his position so he could replace his tongue with two fingers, curling them inside to rub the spot that always made Papyrus’s walls clamp down around him. “i wanna make you cum so hard you’ll think these stars are real” 

“Oh Gosh-But-But- You-”

“believe me, i’m getting mine-” Sans gave his brother a lecherous grin before dropping his tone until it sounded like he was growling his words. “-and we’re not gonna be done anytime soon”

“Oh… _Oh!_ Ok! Then Please- Ah-Oh-Oh My God!” Papyrus’s whole body locked up when Sans started to tongue his clit in tandem to the fingers he was thrusting inside him. “Right There! Sans I’m Gonna-I’m Gonna Cum!” 

A few more jerks while ridding Sans’s face pushed Papyrus over with a long drawn out moan. Sans was sure he’d never seen his brother cum that hard, not to mention how turned on he was at actually hearing Papyrus saying it. He had to use the hand he’d been using to steady Papyrus to grab his cock hard so that he didn’t cream his pants. Of course the contact made him moan into Papyrus and give him aftershocks almost as strong as the real thing. Sans knew he couldn’t wait much longer to do more, but was pleasantly surprised when Papyrus’s own hands were pawing at his shorts as soon as he stood up.

Papyrus was too far gone not to moan himself when he took Sans’s throbbing cock into his hands, sounding like he was getting just as much pleasure as if he was being touched. Sans could hardly believe what was happening, his hazy eyelights flicking from Papyrus’s hand moving on his shaft to the still dripping pussy between his brother's legs. He realised now why Papyrus had such a hard time at first with letting go, Sans was struggling with hearing his own voice while Papyrus was handling him after so long of holding back. The best he had been able to manage was a few curses under his breath as Papyrus gradually worked up his pace, the amount of magic leaking out of his head making him shiver from the sensations. He might have been quiet but luckily Papyrus was just as eager to encourage Sans as he had been to him. 

“Please, Brother I Want To Hear You-” Papyrus whispered while he traced a hand over Sans’s chest, the light pressure to his already sensitive bones making him groan. Papyrus’s eyes were sparkled in wonder, knowing just how to push Sans’s buttons by leaning into his innocent nature. “Do I Make You Feel Good Brother?” he asked, lidding his sockets in place of batting the eyelashes he didn’t have. 

Sans was turning into putty in his brother’s hands. It didn’t matter that he was being manipulated, Sans was ravenous for everything his brother gave him. He had to grab Papyrus for support as his whole body was shaking from the moans he’d been holding back. It was obvious Papyrus was enthralled by the effect he had on him with the way his hand sped up and the way he pressed their foreheads together with a dreamy sigh. With their height difference Papyrus was leaning on Sans just as much as he was, creating a counterbalance that had their heavy breathing syncing up. It was just as sweet as it was hot and with Sans rapidly giving into his more primal urges he realised that their proximity barely satisfied his need to be as close to Papyrus as possible. 

“fuck-fuck paps your so goddamn good-oh shit-” Sans grunted and stilled his brother’s hand with his own even though he couldn’t stop the jerking of his hips “-we need to stop-i wanna be inside you. please, _please_ , bro i need it.”

Papyrus whined at Sans’s tone and did all he could to try and get Sans into position. As Sans picked his brother up with the aid of blue magic he realised that they had somehow moved to a spot on the wall closer to the waterfall. The additional noise was going to be helpful with how hard he needed to fuck Papyrus into the wall, completely worth them getting even wetter. Not that Papyrus needed any help in a particular area. By the time he wrapped his long limbs around Sans and their bare magic finally touched Sans almost dropped him from the feeling of his wetness sliding across his shaft. He relied mostly on the blue magic to make sure that Papyrus was at the right angle on the wall, the glow lighting up the area and bouncing off the pouring water. It lit Papyrus’s porcelain bones up perfectly and for the second time that night Sans counted his lucky stars that he had ended up with someone so breathtaking as his brother.

Sans was looking at his brother’s beautiful face when he pushed his cock inside, the look in Papyrus’s eyes reaching down into his chest and taking a firm hold of his soul.

While usually they would spend time building up their pace to make sure they could handle it without any outbursts today was not the day to take things slow. By the time Sans had fully seated himself inside Papyrus he was clawing at his back and whining for more. Just like earlier when he agreed to the picnic, there was no way in hell Sans was going to say no. He fucked Papyrus into the wall like his life depended on it. With the way Papyrus was whimpering with each thrust it probably did. Sans was kicking himself for not figuring out something earlier, it was a mortal sin to keep Papyrus quiet when he was so naturally responsive. If Sans heard nothing but his brother’s moans for the rest of his life he would die happily. 

Finally being able to enjoy this experience together fully, without any restrictions, made everything feel even more amazing. Sans always felt connected to Papyrus when they were intimate, but now it felt like they were completely in sync with each other. It was a domino effect; Sans would thrust harder just when Papyrus needed it, making his inner walls flutter around his cock and start driving them both towards the edge. Sans couldn’t get enough of the way Papyrus’s pussy was gripping every inch of him like he never wanted to let go. The feeling was definitely mutual. 

“holy shit- you feel so fucking good-oh gods-” Sans was so far gone that he knew he was barely making sense. But he wanted more, he wanted to say all the things he’d never had the chance to and hear Papyrus answering his calls. “tell me how much you like me fucking you like this.”

“It Feels- I-I-” Papyrus’s eyes snapped fully open as a wave of pleasure left him trembling before he cried out and the last bit of his own self-control snapped too. “It Feels Amazing Sans-Ah!- I Love This- I- I Love You!”

Sans choked on the water that fell into his open mouth as his pace faltered. He was expecting Papyrus to have blushed at his vulgarity or better yet gone along with it and said something filthy so the confession caught him completely off guard. 

Had papyrus really just said the one thing Sans wanted so much that it felt like his soul was going to burst?! 

Sans’s voice was cracked and horse as his mind tried to catch up, unable to process what was happening. “paps-i- are you sure?”

For a moment Sans was sure he’d misheard him but Papyrus’s face showed no lies. In fact, he was smiling as he repeated the words Sans would never get tired of hearing.

“I Love You So Much Sans”

“gods- fuck- i love you too,” Sans wiped the stray tear that rolled down Papyrus’s cheek, knowing that his own sockets were also brimming with magic “more than anything in the world”

Papyrus giggled happily before pulling Sans forward and capturing his mouth with a kiss. It was a little more difficult to keep his balance but with the aid of the magic he still managed to have summoned Sans was able to keep Papyrus steady as he kissed him hard. Then he started to move in him again. It wasn’t intentional, Sans would have happily just kissed Papyrus after a confession like that, but their bodies had other plans. The pace they set this time was slow and deep and all their noises were swallowed by their kiss. It didn’t matter that they’d come here to be loud. Not anymore. Nothing mattered but the building feeling of ecstasy between them that was the culmination of their love. The whole underground could crumble down around them at that moment and they wouldn’t care. 

Not while they had each other. 

“Sans- I- I'm- Again- Please I Need You To- Oh My-Ah!” Papyrus all but screamed when they finally parted, the build-up to his peak making his whole body shake. 

Sans wasn’t far behind him, his pace was starting to stutter as his hands gripped Papyrus’s iliac crest hard enough to leave marks. He wanted Papyrus to be feeling this for days after, he never wanted either of them to forget how incredible this moment was. 

“fuck pap that's it cum for me- ah fuck- please cum on my cock- i wanna fucking feel it- holy shit-”

Sans felt his eye lights disappear when Papyrus’s pussy clamped down around his cock for the final time. He thrust in as deep as he could with a loud groan as his brother’s walls milked him of his own climax. They hadn’t even finished cumming when they started kissing again, somehow managing to sink to the floor with Sans still buried inside of him. They held each other as water cascaded around them, making their makeouts sloppy but with no less passion than before. Sans at first thought they were shivering from the cold water but when Papyrus shifted in his lap slightly he realised it was from the aftershocks still tingling across their bones. They had both finished minutes ago, but neither of them made any move to disconnect. Like they couldn’t bear for the moment to end.

So they stayed as close as possible, with Sans whispering ‘I love you’ over and over again. He said it for every time he'd held it back in the past, for every time he knew Papyrus needed to hear it, for every time he heard Papyrus say it back. Everything about this moment felt right, like he wasn’t meant to be anywhere but holding Papyrus in their own little haven. Sans wasn’t surprised when he felt a pull in his ribcage and the small inverted heart appear in the space between him.

Bearing his soul wasn’t something Sans would have ever thought to do by himself, but he could feel the way it was singing for Papyrus and coaxing his own to form right next to his. He knew he didn’t need to ask if it this was ok but he still hesitated when moving his shaking hand towards the almost touching souls. Papyrus knew exactly what he was thinking and moved his opposite hand up so he could push his soul the rest of the way forward. The moment their souls touched made Sans’s body burn white hot and he could see that Papyrus felt the same. No, not see, he could _feel_ it through their connection.

Sans wanted to say how much he loved Papyrus again but there was no need. Their souls were communicating all of the things that words didn’t do justice as they started to rock against each other again in the dimly lit cave.


End file.
